Patent application WO-2011/107,669 describes the coupling of a class A analog amplifier, made up of a reference voltage generator and a class D digital amplifier constituting a power voltage generator, which is coupled to the output of the reference voltage generator by an inductance, with which it then forms a current source.
The combination of a class A amplifier and a class D amplifier is intended to create an amplifier with a very high performance and very high linearity.
In practice, several phenomena limit the total performance of such an amplifier, its ability to reproduce high frequencies, and its maximum achievable power. These phenomena are in particular the following.
The switching losses of the MOS transistors of the class D amplifier are proportional to the switching frequency. For this reason, in practice, this frequency cannot significantly exceed 500 kHz for voltages greater than 100 volts.
Furthermore, the value of the output inductance of the class D amplifier must be as small as possible to allow a maximum slew rate thus allowing high frequencies of the audio spectrum to be reproduced. However, decreasing the value of this inductance produces the following two harmful effects:                the current ripple in the inductance of the class D amplifier is inversely proportional to the switching frequency and inversely proportional to the value of the inductance. Yet the high-frequency current ripple is completely absorbed and dissipated by the class A analog amplifier, which causes significant heat dissipation and decreases the performance of the system;        the maximum authorized gain to remain within the stability limits in the case of proportional integral (PI) feedback control of the class D amplifier by the current consumed by the class A amplifier is directly proportional to the value of the inductance of the class D amplifier and inversely proportional to the sum of all of the time lags and delays of the system in particular consisting of the computing time lags of the microcontrollers and time lags inherent to the use of regulation of the PWM type.        
The compromise that must be found between the technological constraints of switching transistors, the bandwidth to be reproduced and the typical impedance of the speakers to be powered leads to excessively low maximum proportional and integral gain values beyond about 10 kHz, reducing the ratio of the current supplied by the class A amplifier to the current supplied by the class D amplifier at 20 kHz to less than one tenth. At low frequencies, such gain problems do not exist, the integrator present in the control system having a very high gain.
The solution implemented in document WO 2011/107,669, consisting of adding the signal representative of the input voltage of the class A amplifier to the control signal of the class D amplifier, aims, without any current in the class A amplifier, after the integrator of the class D amplifier control system converges towards zero, for the voltage across the terminals of the coupling impedance and the current traversing this impedance to be zero irrespective of the input voltage. Yet this condition proves difficult to meet if the complex impedances of the load and the coupling inductance have different arguments, which is generally the case, primarily when the coupling impedance has a very low resistance and a high inductance, while the impedance of the load is essentially resistive.
Under these conditions, the class A amplifier is highly stressed when reproducing high frequencies, in particular exceeding 10 kHz.